Various coatings of non-luminescent particulate materials are already known to be useful when applied as an undercoating for the phosphor layer in both fluorescent-type and mercury vapor-type lamps. In both of said type lamps, the phosphor coating is disposed on the inner surface of the lamp glass envelope in receptive proximity to the ultraviolet radiation being generated by the mercury discharge. The luminous efficiency of such lamps is improved by back reflection of the incident radiation being emitted from the phosphor layer which has permitted reduction in the phosphor coating weight as well as providing color correction said to be attributable to such modified emission behavior. The prior arts reflecting coatings for this purpose are deposited from liquid coating suspensions with requirements upon the non-luminescent particulate material being negligible absorption; that is, high diffuse reflection coefficient for both visible and ultraviolet radiation, a particle size as small as and preferably smaller in size than the particle size of the phosphor coating, and a further requirement of having physical properties which do not become altered during manufacture or life of the mercury vapor lamp. A preferred non-luminescent particulate material for use in this manner is finely divided silica although other diverse materials which do not absorb either incident ultraviolet radiation or visible radiation being emitted by the phosphor include calcium pyrophosphate, barium sulfate, and alumina.